1. Field of the Invention
The present invention relates to a hydraulic motor control system for controlling a hydraulic motor such as a rotating motor or a traveling motor used in a hydraulic working machine such as a hydraulic excavator or a hydraulic crane.
2. Description of the Related Art
A rotating motor used in a conventional hydraulic excavator or hydraulic crane will now be described. FIG. 22 illustrates a circuit for a rotating motor in a conventional hydraulic excavator. In the same figure, the numeral 1 denotes a rotating motor, the numeral 2 denotes a hydraulic pump as an oil pressure source for the motor 1, the numeral 3 denotes an engine for driving the pump 2, the numeral 4 denotes a hydraulic pilot change-over type control valve for controlling the operation of the motor 1, and the numeral 5 denotes a remote control valve (the numeral 5a denotes an operating lever). A pilot pressure proportional to the amount of operation of the lever in the remote control valve 5 is fed to a pilot port of the control valve 4 to control the valve 4. As known well, the control valve 4 has meter-in, meter-out and bleed-off passages. The opening area of each of these passages varies in proportion to a spool stroke to change the flow rates for meter-in, meter-out and bleed-off, whereby the acceleration or deceleration (rotating torque) of the rotating motor 1 changes. The numeral 6 denotes a relief valve and the numeral 7 denotes an auxiliary hydraulic pump for supplying a primary pressure to the remote control valve 5.
In a hydraulic excavator, the moment of inertia varies greatly according to the posture of front attachments (boom, arm and bucket) or an excavating load imposed thereon. If the valve characteristics of the control valve 4 are set on the basis of a large moment of inertia, the rotating torque will become too large at a small moment of inertia, thus causing a fear of sudden acceleration. Besides, the operability is deteriorated, and a sudden acceleration acts also on the operator operating in the cabin, thus exerting a bad influence on the operation of the lever. The resulting vibratory operation of the lever is likely to induce a hunting phenomenon. On the other hand, the above problem can be avoided if valve characteristics are set on the basis of a small moment of inertia. However, the rotating torque becomes too small, resulting in that the excavating work or a pushing or leveling work using a rotating force is impeded.
In the conventional hydraulic excavator, the moment of inertia and the rotating torque are not well balanced, so that a certain moment of inertia causes a state of excessive torque or insufficient torque. Such a problem is not limited to the rotating motor. As to the traveling motor, almost the same problem has occurred heretofore.